Sleep debt is the cumulative effect of not getting enough sleep. A large sleep debt may lead to mental and/or physical fatigue. There is debate in the scientific community over the specifics of sleep debt.

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There is debate among researchers as to whether the concept of sleep debt describes a measurable phenomenon. The September 2004 issue of the journal Sleep contained dueling editorials from two of the world's leading sleep researchers: David F. Dinges and Jim Horne. A 1997 experiment conducted by psychiatrists at the University of Pennsylvania School of Medicine [1] suggested that cumulative nocturnal sleep debt affects daytime sleepiness, particularly on the first, second, sixth, and seventh days of sleep restriction.

Sleep debt has been a subject in health and work psychology courses where it is mentioned that in order for one to "pay back" this debt it is necessary to sleep for half the amount of time owed.[How to reference and link to summary or text] For example, if of a daily recommended 8 hours sleep one sleeps only 4 hours, one will owe 4 hours; therefore 2 hours of sleep can make up for the 4. This rule is oversimplified, as timing and other factors will play a role.

Sleep debt has been tested in a number of studies, most notably by Klerman and Dijk through the use of a sleep onset latency test.[2] This test attempts to measure how easily someone can fall asleep. When this test is done several times during a day, it is called a multiple sleep latency test (MSLT). The subject is told to go to sleep and is awakened after a short period of time to determine the amount of time it took to fall asleep.

However, one does not have to go to a sleep clinic to try this experiment; a home process has been considered: it involves relaxing quietly and alone for a short amount of time. If the feeling of sleep comes fairly easily, one is considered to have sleep debt. Some also suggest that the quality of sleep can have an effect on the level of one's sleep debt.

The Epworth Sleepiness Scale (ESS) is among the tools used to measure potential sleep debt. Specifically, the ESS; created by Australian researchers, is a simple eight item questionnaire with scores ranging 0-24.

A January 2007 study[3] suggests that saliva tests of the enzyme amylase could be used to indicate sleep debt, as the enzyme increases its activity in correlation with the length of time a subject has been deprived of sleep.

The National Geographic Magazine reported the demands of work, social activities, and the availability of 24-hour home entertainment and internet access have caused people to sleep less now than in premodern times.[4] However, Jim Horne, a sleep researcher at Loughborough University, questions such claims. In a 2004 editorial in the journal Sleep, he notes available data suggest the average number of hours of sleep in a 24-hour period has not changed significantly in recent decades among adults.[How to reference and link to summary or text] Comparing data collected from the Bureau of Labor Statistics' American Time Use Survey[5] from 1965-1985[6] and 1998-2001,[7] shows that the median amount of sleep, napping, and resting done by the average adult American has changed by less than 0.7%, from a median of 482 minutes per day from 1965 through 1985, to 479 minutes per day from 1998 through 2001. Furthermore, the editorial suggests that there is a range of normal sleep time required by healthy adults, and many indicators used to suggest chronic sleepiness among the population as a whole do not stand up to scientific scrutiny.